Re: Steppers or servos

[Jon Elson <jmelson-at-artsci.wustl.edu>]

David Preston wrote:

> I am curious to know whether steppers or servo motors are better
> suited to EMC. Perhaps someone knowledgable in this area could explain
> the pros and cons of each, I also have ready access to numerous
> smaller servo motors and was wondering if it's possible, and if so,
> how difficult it would be to wire several smaller motors in parallel
> driving a common leadscrew. Last but not least, can someone please
> tell me if EMC can easily run a fourth and fifth axis (rotary i.e. an
> indexing head) as well as xyz?

Well, it's not EMC really.  EMC is one of the few programs that can run
EITHER
servos or steppers.  There is a limit on step rate with EMC, now.
Servos don't
suffer this limitation.  If you want very smooth, accurate parts, I
recommend
servos.  A good servo amp will keep the motion smooth down to extremely
low velocities (like .01"/min) without any stair-steps.

Wiring motors in parallel is no big deal, but COUPLING them mechanically

in parallel is more difficult.  I really don't think it is practical.
Surplus servo
motors in larger sizes are available at low cost, if you look
carefully.  You
might try eBay, where a lot of servo and motion control stuff is sold.

EMC will soon have capability of running up to 6 axes of mixed linear/
rotary type, as well as Stewart platforms in 6 axes, I'm told.
Some of the software components that prevented this have now
been changed to allow the extra axes.  There is one component
left, the kinematics routines, and work is underway now to add the
extra axes to that.  There will probably be several versions made,
and either scripts or configuration files that will allow you to set up
the axes and offsets for the machine.  This should allow the complex
calculations involving the distance between the centers of rotation
of the rotary axes to be performed, so the XYZ-AB(C) coordinates
can be entered in workpiece coordinates, without regard for the
physical configuration of the stacked axes on the machine.  The
kinematics routines will compute the necessary movements to
put the tool where the programmer wants it on the workpiece.

When this is all working, it will move EMC a lightyear ahead
of the other low-cost CNC's, which don't do this for you.

Jon